1. Field of the Invention
The invention relates to birefringent filters including tunable birefringent filters which incorporate fixed retarder elements and variable retarder elements; and to multispectral imaging systems using such filters.
2. Description of the Related Art
Filters based on liquid crystal variable retarder elements are described in U.S. Pat. No. 5,247,378. That patent describes methods for determining the proper drive voltage to apply to a liquid crystal cell in order to produce a desired optical retardance, based on its electrical capacitance properties. It also describes a tuning method which reduces or eliminates drift in the center wavelength of a birefringent filter, wherein a control means determines a target retardance value for the variable retarder element, based on the temperature of the filter assembly and an estimate of the temperature coefficient of the fixed retarder elements.
Commercial liquid crystal tunable filters (LCTFs) are available from Cambridge Research & Instrumentation, Inc (Woburn, Mass.) under the trade name VariSpec filters.
It is typical that tunable filters use an algorithm where the variable retarder is set to a value such that the combination of the fixed retarder element and the variable retarder element produce a net retardance of Nλ; or of (N+½)λ; or of (N+¼)λ or (N+¾)λ; depending on the filter design. In these expressions, N is an integer and λ is a wavelength of interest such as the wavelength for which the desired transmission maximum is sought. For a given design, one of these expressions may be sought because it corresponds to a peak in the transmission function for the overall filter, with the choice of expression depending on whether the retarder is in a Lyot stage with parallel polarizers, crossed polarizers, or some other arrangement such as an Evans split-element stage.
Liquid crystal variable retarders have limited range of adjustment, but an adjustment range of λ is sufficient to ensure there is always at least one setting available for which the combined retardance meets the appropriate N, N+½, N+¼, or N+¾ criterion.
The value of N often depends upon the tuning wavelength λ, and this dependence can be denoted N(λ). The filter bandwidth depends on tuning wavelength λ, and this dependence can be denoted B(λ).
Optical filters are operated in a variety of environments, including environments where temperature changes may occur. It is desirable that a tunable filter exhibit athermal response, meaning that none of its properties are altered when the temperature changes.
Multispectral imaging systems based on a combination of tunable birefringent filters, imaging detectors, and relay optics are sold commercially by Cambridge Research & Instrumentation, Inc (Woburn, Mass.). The spectral properties of the system, such as bandwidth, center wavelength, and throughput, are principally determined by the birefringent filter.